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fix_viscous.html
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rLAMMPS lammps
fix_viscous.html
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<HTML>
<CENTER><A
HREF =
"http://lammps.sandia.gov"
>
LAMMPS WWW Site
</A>
-
<A
HREF =
"Manual.html"
>
LAMMPS Documentation
</A>
-
<A
HREF =
"Section_commands.html#comm"
>
LAMMPS Commands
</A>
</CENTER>
<HR>
<H3>
fix viscous command
</H3>
<H3>
fix viscous/cuda command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
fix ID group-ID viscous gamma keyword values ...
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"fix.html"
>
fix
</A>
command
<LI>
viscous = style name of this fix command
<LI>
gamma = damping coefficient (force/velocity units)
<LI>
zero or more keyword/value pairs may be appended
<PRE>
keyword =
<I>
scale
</I>
<I>
scale
</I>
values = type ratio
type = atom type (1-N)
ratio = factor to scale the damping coefficient by
</PRE>
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
fix 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Add a viscous damping force to atoms in the group that is proportional
to the velocity of the atom. The added force can be thought of as a
frictional interaction with implicit solvent, i.e. the no-slip Stokes
drag on a spherical particle. In granular simulations this can be
useful for draining the kinetic energy from the system in a controlled
fashion. If used without additional thermostatting (to add kinetic
energy to the system), it has the effect of slowly (or rapidly)
freezing the system; hence it can also be used as a simple energy
minimization technique.
</P>
<P>
The damping force F is given by F = - gamma * velocity. The larger
the coefficient, the faster the kinetic energy is reduced. If the
optional keyword
<I>
scale
</I>
is used, gamma can scaled up or down by the
specified factor for atoms of that type. It can be used multiple
times to adjust gamma for several atom types.
</P>
<P>
IMPORTANT NOTE: You should specify gamma in force/velocity units.
This is not the same as mass/time units, at least for some of the
LAMMPS
<A
HREF =
"units.html"
>
units
</A>
options like "real" or "metal" that are not
self-consistent.
</P>
<P>
In a Brownian dynamics context, gamma = Kb T / D, where Kb =
Boltzmann's constant, T = temperature, and D = particle diffusion
coefficient. D can be written as Kb T / (3 pi eta d), where eta =
dynamic viscosity of the frictional fluid and d = diameter of
particle. This means gamma = 3 pi eta d, and thus is proportional to
the viscosity of the fluid and the particle diameter.
</P>
<P>
In the current implementation, rather than have the user specify a
viscosity, gamma is specified directly in force/velocity units. If
needed, gamma can be adjusted for atoms of different sizes
(i.e. sigma) by using the
<I>
scale
</I>
keyword.
</P>
<P>
Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature. The
<A
HREF =
"fix_langevin.html"
>
fix
langevin
</A>
command does this. It has the same
viscous damping term as fix viscous and adds a random force to each
atom. Hence if using fix
<I>
langevin
</I>
you do not typically need to use
fix
<I>
viscous
</I>
. Also note that the gamma of fix viscous is related to
the damping parameter of
<A
HREF =
"fix_langevin.html"
>
fix langevin
</A>
, except that
the units of gamma are force/velocity and the units of damp are time,
so that it can more easily be used as a thermostat.
</P>
<HR>
<P>
Styles with a
<I>
cuda
</I>
suffix are functionally the same as the
corresponding style without the suffix. They have been optimized to
run faster, depending on your available hardware, as discussed in
<A
HREF =
"Section_accelerate.html"
>
this section
</A>
of the manual. The accelerated
styles take the same arguments and should produce the same results,
except for round-off and precision issues.
</P>
<P>
These accelerated styles are part of the USER-CUDA package. They are
only enabled if LAMMPS was built with that package. See the
<A
HREF =
"Section_start.html#start_3"
>
Making
LAMMPS
</A>
section for more info.
</P>
<P>
You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the
<A
HREF =
"Section_start.html#start_6"
>
-suffix command-line
switch
</A>
when you invoke LAMMPS, or you can
use the
<A
HREF =
"suffix.html"
>
suffix
</A>
command in your input script.
</P>
<P>
See
<A
HREF =
"Section_accelerate.html"
>
this section
</A>
of the manual for more
instructions on how to use the accelerated styles effectively.
</P>
<HR>
<P><B>
Restart, fix_modify, output, run start/stop, minimize info:
</B>
</P>
<P>
No information about this fix is written to
<A
HREF =
"restart.html"
>
binary restart
files
</A>
. None of the
<A
HREF =
"fix_modify.html"
>
fix_modify
</A>
options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various
<A
HREF =
"Section_howto.html#howto_15"
>
output
commands
</A>
. No parameter of this fix can
be used with the
<I>
start/stop
</I>
keywords of the
<A
HREF =
"run.html"
>
run
</A>
command.
</P>
<P>
The forces due to this fix are imposed during an energy minimization,
invoked by the
<A
HREF =
"minimize.html"
>
minimize
</A>
command. This fix should only
be used with damped dynamics minimizers that allow for
non-conservative forces. See the
<A
HREF =
"min_style.html"
>
min_style
</A>
command
for details.
</P>
<P><B>
Restrictions:
</B>
none
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"fix_langevin.html"
>
fix langevin
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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